By about three billion years ago, Mars was a wasteland — a world far drier than almost any desert on Earth. The rivers that carved long, deep channels had quit flowing, and a possible global ocean had long since evaporated.

Even so, there’s evidence that watery oases have popped up on the surface throughout the planet’s long history.

An example is a region near the end of the long canyon system known as Valles Marineris. An orbiting spacecraft has revealed a couple of depressions that are coated with a type of clay — an amalgam of minerals that form in water.

Researchers from the Planetary Science Institute studied observations of the region made by Mars Reconnaissance Orbiter. The observations show layers of clays around two deep troughs.

The researchers speculate that nearby volcanic activity could have melted ice below the surface, which bubbled up to fill the troughs. The process could have repeated several times, repeatedly forming lakes. The last episode happened about two billion years ago.

The presence of water means that these troughs would have been comfortable homes for microscopic life. The region is far too rugged for current landers or rovers, though, so we probably won’t be able to search for evidence of life there for a long time to come.

Look for Mars following the gibbous Moon across the sky tonight. Mars rises to the lower left of the Moon, and looks like a bright orange star. More about Mars and the Moon tomorrow.